The patent document CH-A 590 518 has already described a system for the determination of times separating the passages of moving objects to the right of a reference line substantially perpendicular to the trajectory. This system consists of employing a television camera equipped with a cathode ray tube, which camera is directed onto the reference line, recording the signals provided by said camera and simultaneously signals provided by a timekeeper and reproducing the signals with the aid of a monitor. In order to accomplish this, one employs a camera by means of which one effects a unidirectional linear scan coinciding with the reference line and a reader effecting a linear bidirectional scan in a manner such that the successive scans of the reference line by the camera are spread out over the screen of the reader in a direction perpendicular to that in which the unidirectional scans are effected, thus in the direction of the trajectory of the moving objects. The document mentions that this camera may be of a conventional type, where however the two scans are permuted in a manner such that the more rapid scan is effected in a vertical direction and a slower scan has been suppressed. As a variant it is said that one will employ preferably a diode camera of the type referred to as solid state in the place of a cathode ray tube in order to avoid marking the tube or indeed the rapid deterioration of the latter.
This system is illustrated on FIGS. 1 and 2 of the present description. The moving object 2 is displaced at a speed V.sub.m in front of lens 1 of the camera. Behind the lens is found a unidimensional photosensitive arrangement 5 using a solid state detector, here further called a CCD bar. The image of the moving object runs past at the speed Vi in front of arrangement 5. FIG. 2 shows how this arrangement is formed which includes a row of elementary pixels 6 arranged side by side. The incident light coming from the reference line to be captured produces charges on each of the detectors, which charges represent the intensity profile of a line of the image at a given instant. These charges are periodically transferred in the sense of arrows 9 into a shift register 7 bearing as many elements 8 as there are pixels 6. A clock signal 12 at TV frequency empties the contents of the line towards amplifier 10 in the form of a video signal 11. These video signals are next memorized, then visualized in a manner to represent, in the form of an image, the development in time of the line under observation (finish line for instance).
Several unidirectional arrangements are presently available on the market. They are all equipped with a substantial number of pixels ( &gt;1000) to assure high resolution. For further details on this subject, reference may be had to the technical publications of the manufacturers, for instance to the publication concerning the arrangement GH 7801 A of Thomson-CSF. Complete cameras may even be obtained from the Fairchild Company under the reference CCD 1100C to 1500C or again from the i2S Company (Bordeaux, France) under the denomination iDC 133.
In the system which has just been described, it has been seen that the incident light produces charges on the row of detectors 6 which charges are periodically transferred towards the output 10 via a shift register 7. Here the transfer frequency is fixed since it is tied to an ordinary television standard. In effect, in the cited document is noted a frequency of the images which is 25 units per second, each half image lasting 20 milliseconds. From this fact the system described has the merit of employing standard material as far as concerns the camera as well as the monitor and the recorder. It is thus inexpensive.
The system described unfortunately presents at least two major disadvantages which determine that it has never been employed in practice and remains at the stage of a prototype.
The first difficulty concerns the time of exposure of the pixels which is very small. Effectively, according to the European standard, a half image is explored in 20 ms (50 Hz) and each half image includes 312.5 lines. It follows that the duration of a line is equal to 20/312.5 0.064 ms=64 .mu.s and that the scan frequency is 1/64 .mu.s, i.e. 15,625 lines per second. Thus, in employing a unidimensional photosensitive arrangement with standard TV sweep, each pixel will be excited only during 64 .mu.s per scan. This represents an extremely short time which limits the domain of application of the arrangement to scenes presenting substantial lighting, since for average illumination the signal gathered will not emerge at all or very little from the background noise, at least with the means presently available.
The second difficulty concerns the deformation of the images collected. It will be understood in effect that to obtain a non-deformed image, it will be necessary that the speed of refreshing of the photo sensitive arrangement given by the clock signal 12 (see FIGS. 1 and 2) correspond to the speed Vi of the image running past on said arrangement. The image collected will be compressed if the speed Vi is greater than the refreshing speed or on the contrary, will be dilated if the speed Vi is less than said refreshing speed. An example drawn from practical situations will facilitate understanding of the problem which is posed.
The CCD bar as mentioned hereinabove includes a multiplicity of pixels of a substantially squared off surface, the side of which measures substantially 13 .mu.m. With the standard TV at 625 lines, these 13 .mu.m are scanned as already been said in 64 .mu.s, which corresponds to the image of a moving object which would be displaced at the speed Vi of: ##EQU1## In this case, the scan frequency is adapted to the speed of the race and the images reproduced appear without deformation. Now this value corresponds to the speed Vi of an automobile race running in front of the CCD bar. Should one now wish, with the same apparatus, to capture the images of an athletics race of which the speed of the image Vi may be estimated to be 26 mm/s, the image collected will be strongly dilated in width deforming the shapes of the athletes to the point of rendering them unrecognizable. If one wanted thus to obtain an image without deformation of the athletics race, it would be necessary to lower the scan frequency of the bar which correspondingly will increase the time during which the pixels of the bar are exposed. In taking the speed of 26 mm/s and a pixel of 13 .mu.m on one side, one may calculate the exposure time, then the scan frequency to be applied to the bar. The exposure time is: ##EQU2## and the scan frequency is 1/500 .mu.s=2,000 lines per second. To take another example, the exposure time and the scan frequency would be respectively of 3.25 ms and 307 lines per second if one considered a boating trial where the speed of the image Vi running in front of the bar is on the order of 4 mm/s.
It follows from what has just been said that in order to obtain a non-deformed image of the course to be timed, it is indispensable to adapt the scan frequency of the bar to the speed of the image running past onto such bar in the same manner as one adapts the speed of film in a system employing a film running past behind a slot (process of photo-finish described in the patent document CH-A-399 028). In the arrangement set forth in patent document CH-A 590 581 cited hereinabove, this adaptation is effected only for high speed races such as for automotive vehicles. It will be understood that in order to capture the image of an athletics race, not only must the scan frequency of the bar be reduced to the values indicated hereinabove, but further it is necessary to have available a reader (monitor, video recorder) the scan frequency of which is tuned and synchronous with that which explores the bar. This will never be the case if one wishes to employ a commercial reader conceived for a single frequency (15,625 lines per second) and established once and for all. In order to resolve this problem, it has been proposed to employ a reader having an adaptable frequency. One could also propose that the processing and storing of images be effected in a PC. Whatever be the chosen solution, it will be necessary to employ a complex and onerous collection of apparatus.
The patent document EP-A-O 223 119 proposes however an apparatus for capturing the image of sporting races including a pulse generator for controlling the transfer of charges from one sensor to the other, in a manner such that the speed of this transfer is made to correspond to the speed of the image of the race which is passing in front of the arrangement. No detail however is given concerning the manner in which the image is reconstructed the text being satisfied to explain that this reconstitution is brought about according to techniques known in television, the line frequency being synchronized with the charge transfer frequency, which implies a monitor and a video recorder which are non-standard.
The patent document EP-A-0 207 675 likewise proposes a video recording apparatus for sporting races including a one dimensional sensor in the form of a bar. The signal gathered at the output of the sensor is converted by an A/D converter into a series of image elements which are stored in a video memory in order to form a plurality of images arranged end to end. This system is however limited to the capacity of the memory, typically to 16 TV images as the description indicates. Such is due to the fact that in this system there are not two buffer memories alternately working, one in reading and the other in writing as is the case in the invention which will be described hereinafter.
The patent document US-A-4 133 009 proposes for its part two buffer memories working alternately. However, the capacity of these memories is determined by an entire TV image from whence there does not result difficulty in reconstituting the TV image in a standard monitor and video recorder. This system cannot be applied to capturing the image of races along with a time scale for one arrives at ambiguities in passing from one image to the other (overlapping) since at no moment is there to be found on the screen the juxtaposition of a plurality of image portions at the same time, so as to bring about a temporal continuity between one of the portions and the portions which precede and follow said portion, as is proposed by the present invention.
If this invention employs several of the characteristics as described in the three documents which have just been discussed, it is with an entirely different purpose from that pursued by said documents. The problem to be solved here results from the fact that as the frequency of image acquisition by the bar is not synchronized with the frequency of visualization, one proceeds with generating portions of an image employing buffer memories, such image portions being next stored in an image memory, then read in a discontinuous fashion by jumping over portions of the image in synchronism with a standard TV sweep. Thanks to this arrangement, the images are recorded and visualized by means of a standard commercial recorder and monitor at a scan frequency which is fixed and standardized.